Will,
First of all I have said before and will repeat it, I detest the term "roofing
filter." That said, by the generally accepted definition, you are wrong. See
Elecraft's take on this:
http://www.elecraft.com/K3/Roofing_Filters.htm
If you will think in Wayne's terms, the post-mixer filter is a "protective"
filter, not a mode-specific filter. So the question becomes, how much
protection is necessary? In Elecraft's case, quite a lot, IMHO. With its QRP
DNA, Elecraft uses post crystal filter circuitry that minimizes current
consumption. The trade off for this is the need for a bank of pricey crystal
filters to limit the frequencies that the circuitry is exposed to.
Now what if the subsequent circuitry doesn't require this much protection
because it is more robust? We now have direct-sampling radios that can digitize
a whole ham band with good performance. If the BW was limited to 10-15 kHz in an
up conversion configuration they should be even better. The limitation now
becomes LO phase noise, but newer synthesizer designs overcome that obstacle.
Another thing to note is that IMD in crystal filters is reported to be inversely
proportional to BW. So a wider filter might actually be better from that
perspective. Some Elecraft filters exhibit passive IMD BTW.
Wes N7WS
On 6/14/2018 8:01 AM, WILLIE BABER wrote:
Hello Wes,
I took a look. Both designs are using the idea of "roofing filter" to refer to
up-conversion radios similar to the use of up-conversion 3khz filters as roofing filters
in Icom radios.
"Roofing filter" (a mode specific filter after the first mixer including narrow cw
filters) only makes sense in the context of the history of superhet design and in particular the
use of one broad 15 khz first I-F (so that all modes may pass through it) typical of all Japanese
radios until recently. Calling a 45 mhz filter at the first I-F a "roofing filter" as
noted in the info you sent entirely misses the point of what roofing filter means. Or, to put it
another way, all Ten-Tec radios had roofing filters in them (and were ssb and cw only) well before
the term roofing filter was coined! Which is why an Omni C will out perform any wide (15 khz)
first I-F Japanese radio, even those built well after the 1980 vintage Omni C.
Unless mode specific up-conversion crystal filters can be made and as narrow as 200 hz
(this is possible with down-conversion) then "roofing filter" and up conversion
doesn't make sense historically or in reality.
Actually, Icom says that did it with 1.2khz filter at 64 mhz in the Icom 7851,
though I'm not convinced the filter is that narrow, and 1.2khz is far from the
200hz filter that my K3 has in it (however, the placement of this filter is why
the 7851 is among the best radios in Sherwood's chart, on cw).
It is possible to make very narrow and precise crystal filters as narrow as the
200 hz inexpensively, and this is the point of having multiple roofing filters
at the first I-F. So, this is the origin of the term roofing filter---in
comparison to the barn-door up conversion first I-F.
73, Will, wj9b
CWops #1085
CWA Advisor levels II and III
http://cwops.org/
--------------------------------------------
On Wed, 6/13/18, Wes Stewart<wes_n...@triconet.org> wrote:
Subject: Re: [Elecraft] Field Day rig experience
To:elecraft@mailman.qth.net
Date: Wednesday, June 13, 2018, 3:08 PM
Certainly not to disparage the
K3(S) architecture (I have two of them) there is
nothing inherently wrong with an up-conversion
receiver, if modern hardware is used.
See:https://martein.home.xs4all.nl/pa3ake/hmode/g3sbi_intro.html
and my friend Cornell's,
Star-10 transceiver.
https://pdfs.semanticscholar.org/eb33/5c12858779a653d9b9b93ca20120aebb7616.pdf
Wes N7WS
On 6/13/2018 11:38 AM, WILLIE BABER
wrote:
> Robert is talking about the
crystal filters, also known as roofing filters now-days,
that are typically placed after the first mixer (I
mistakenly typed "ahead" but I meant
"after" as Robert notes), though there is a post
amp and NB before these filters in K2 and K3.
>
> The idea is that a
crystal filter right after the first mixer gives high
dynamic range because high selectivity comes before the
receiver has developed stages of gain that otherwise could
cause blocking or IMD, especially when selectivity is
postponed to the second mixer while ignoring gain
distribution in prior stages of the receiver. This basic
idea was popularized in Solid State Design for the Radio
Amateur, and it was applied to Ten-Tec radios for decades
(at a 9 mhz I-F).
>
>
Roofing filter gets defined in relationship to Japanese
radios that had up conversion 15 khz filters at the first
I-F, and generally lower dynamic range as a result, (but you
got all modes, general coverage, and optional crystal
filters at the second I-F).
>
> Good for everyone radios.... but with
lower dynamic range and phase noise from the early
synthesizers. This is why Ten-Tec radios were so popular
among contesters, especially Omni V and VI (modified with a
narrow cw filter at the first I-F).
>
> 73, Will, wj9b
>
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